Hmm, let's see if I can make it easier...
From: The Sound of Music (1965)
Two drifters off to see the world
From: Breakfast at Tiffany's (1961)
Put the blame on Mame, boys
From: Gilda (1946)
Oh! I really think you're swell!
From: The Parent Trap (1960)
Now you sing, Schmidt!
(Starts around 4:20)
From: Lilies of the Field (1963)
I don't care if it rains or freezes 'long as I got my Plastic Jesus
From: Cool Hand Luke (1967)
I spent ages trying to think of other instances of guitars (and banjitars ;D) in films, but whilst I could think of many other instruments, guitars just weren't there. If you can think of any more, PLEASE let me know because I know there must be so many obvious ones that I've left off.